Otherwise the formal spelling of,
redonkadonk, is derived from both
redonkulous and badonkadonk; hence
coupling verbal and visual absurdity together in order to portray something (emotionally and/or physically) unfathomably disturbing, amazing, or - in many cases - both.
Did I tell you about the time I saw
Pat Robertson at a strip club getting a
lapdance and screaming, "Hallelujah!!"? ...Yeah...
Redonk-a-donk.